docs: def _plane2col(plane):
      '''take a string like 'xy', and return the indices from COLS.*'''
      planes = ('xy', 'yx', 'xz', 'zx', 'yz', 'zy')
      assert plane in planes, 'No such plane found! Please select one of: ' + str(planes)
      return (getattr(COLS, plane[0].capitalize()),
              getattr(COLS, plane[1].capitalize()), )